1. Field
The present invention relates generally to actuators, and more particularly to mechanical stepper motor like actuators. In particular, the present invention relates to mechanical stepper motors with multiple step sizes.
2. Prior Art
Since the introduction of 155 mm guided artillery projectiles in the 1980's, numerous methods and devices have been developed for the guidance and control of subsonic and supersonic gun launched projectiles. The majority of these devices have been developed based on missile and aircraft technologies, which are in many cases difficult or impractical to implement on gun-fired projectiles and mortars. This is particularly true in the case of actuation devices, where electric motors of various designs have dominated the guidance and control of most guided weaponry.
In almost all guided weaponry, such as rockets, actuation devices and batteries used to power the same, occupy a considerable amount of the weaponry's internal volume. In recent years, alternative methods of actuation for flight trajectory correction have been explored, some using smart (active) materials such as piezoelectric ceramics, active polymers, electrostrictive materials, magnetostrictive materials or shape memory alloys, and others using various devices developed based on microelectromechanical (MEMS) and fluidics technologies.
In general, the available smart (active) materials such as piezoelectric ceramics, electrostrictive materials and magnetostrictive materials need to increase their strain capability by at least an order of magnitude in order to become potential candidates for actuator applications for guidance and control, particularly for gun-fired munitions and mortars. In addition, even if the strain rate problems of currently available active materials are solved, their application to gun-fired projectiles and mortars will be very limited due to their very high electrical energy requirements and the volume of the required electrical and electronics gear. Shape memory alloys have good strain characteristics but their dynamic response characteristics (bandwidth) and constitutive behaviour need significant improvement before becoming a viable candidate for actuation devices in general and for munitions in particular.
The currently available and the recently developed methods and devices or those known to be under development for guidance and control of airborne vehicles such as missiles, have not been shown to be suitable for gun-fired projectiles and mortars. In fact, none have been successfully demonstrated for gun-fired guided munitions, including gun-fired and mortar rounds. This has generally been the case since almost all the available guidance and control devices and methodologies suffer from one or more of the following major shortcomings for application in gun-fired projectiles and mortars:                1. Limited control authority (generated force or torque) and high speed actuation capability (dynamic response characteristics or “bandwidth”), considering the dynamics characteristics of gun-fired projectiles and mortars.        2. Reliance on battery-based power for actuation in most available technologies and the requirement of a considerable amount of electrical power for their operation.        3. The relatively large volume requirement for the actuators, batteries and their power electronics.        4. The high cost of the existing technologies, which results in very high-cost rounds, thereby making them impractical for large-scale fielding.        5. Survivability problems of many of the existing devices at high-g firing accelerations and reliability of operation post firing, particularly at very high setback accelerations of over 60,000 Gs.        6. Relative technical complexities involved in their implementation in gun-fired projectiles and mortars.        
A need therefore exists for actuation technologies that address these restrictions in a manner that leaves sufficient volume onboard munitions for sensors, guidance and control and communications electronics and fusing as well as the explosive payload to satisfy lethality requirements.
Such actuation devices must consider the relatively short flight duration for many of the gun-fired projectiles and mortar rounds, which leaves a very short time period within which trajectory correction has to be executed. Such actuation devices must also consider problems related to hardening components for survivability at high firing accelerations and the harsh environment of firing. Reliability is also of much concern since the rounds need to have a shelf life of up to 20 years and could generally be stored at temperatures in the range of −65 to 165 degrees F.
In addition, for years, munitions developers have struggled with placement of components, such as sensors, processors, actuation devices, communications elements and the like within a munitions housing and providing physical interconnections between these components. This task has become even more prohibitive considering the current requirements of making gun-fired munitions and mortars smarter and capable of being guided to their stationary and moving targets, therefore requiring high power consuming and relatively large electrical motors and batteries. It is, therefore, important for all guidance and control actuation devices, their electronics and power sources not to significantly add to the existing problems of integration into the limited projectile volume.
In numerous munitions and other similar applications, the mechanical stepper motor type actuator is required to provide a limited number of linear or rotary or the like motion steps. For example, when used to actuate fins or canards or the like on guided munitions, a limited number of fin or canard positioning, for example five fin or canard positioning, is generally sufficient for proper operation of the guided munitions guidance and control. For example if canards can be rotated in either direction in two steps of up to 10-15 degrees and up to 20-30 degrees, the munitions guidance and control operation can be successfully executed.
In addition, in many machines, actuation devices such as the present mechanical stepper motor type actuation devices are desired to provide more than one and preferably “courser” and “finer” step sizes.
It is appreciated by those skilled in the art that linear (rotary) actuators are generally used to apply linear (rotary) motion, i.e., with minimal applied force (torque), or a combination of linear (rotary) motion and a considerable amount of force (torque). When the level of applied force (torque) to the shuttle is negligible, then a simple friction pad or the like may suffice to ensure that the shuttle would not undergo unwanted motion as an actuating pin is being inserted into a receiving pocket of the actuator shuttle or while an actuated actuating pin is being withdrawn from a shuttle pocket. When the level of applied force (torque) to the shuttle is significant, then unwanted shuttle motion must be prevented while an actuating pin is being withdrawn and the next actuating pin tip has not yet made contact with the corresponding shuttle pocket surface.
A need therefore exists for linear as well as rotary mechanical stepper motor type actuator designs for munitions and similar applications in which a limited number of motion steps are required for proper operation (in the case of munitions the proper operation of the guidance and control function) of the device. It is appreciated that by limiting the number of actuation steps in a mechanical stepper motor type actuation device, the actuation device can be simpler in design and be made to occupy a smaller volume.
A need therefore also exists for linear as well as rotary mechanical stepper motor type actuation devices that are capable of applying controlled force rather than motion steps to the actuated system.
A need therefore also exists for preventing the shuttles of linear as well as rotary mechanical stepper motor type actuation devices from undergoing unwanted motion due to the applied external forces/torques and the like as the actuating pins are advancing the shuttle in the desired direction by their sequential actuation.